Ion conductive polymers find application in several fields of technology, for instance, in electrochemical applications such as proton-exchange fuel cells, batteries, ion exchange membranes and sensors.
It is generally known that emulsions can be formed from a mixture of oil, water and surfactant. A thermodynamically unstable emulsion appears milky while a thermodynamically stable emulsion appears as a transparent colloidal dispersion of oil and water stabilised surfactant, known as a microemulsion.
Microemulsions can have a variety of fluid structures, depending on the components that are comprised in the microemulsion. For example, microemulsions can be in the form of droplets which are swollen with oil and dispersed in water (OW), or they can be swollen with water and dispersed in oil (W/O). Additionally, there may exist bicontinuous microemulsions which have oil and aqueous domains that are randomly interconnected to form sponge-like structures.
The unique fluid structures of microemulsions have been used to provide polymerization loci for monomers. Kuo et al. (Macromolecules, 1987, 20, page 1216) describe the polymerization of styrene in an O/W microemulsion, while Candau et al. (U.S. Pat. No. 4,681,912) describe the polymerization of water soluble monomers in W/O microemulsions.
In order to obtain transparent microporous polymeric materials, various types of reactants have been used in W/O and bicontinuous microemulsion polymer blends. For example, U.S. Pat. No. 5,521,229 discloses the preparation of polymers having a nonporous, bicontinuous structure employing photo-polymerization of microemulsions comprising water, a polar species, a hydrophobic unsaturated monomer, and a polymerizable aliphatic surfactant. Price (U.S. Pat. No. 5,151,217) discloses a bicontinuous microemulsion comprising hydrophobic monomers such as styrene and quaternary ammonium salts as polymerizable surfactant. In addition, European Patent Application No. EP 0 531 005 A2 discloses the use of polar liquids such as alcohols or amides in forming the continuous liquid phase of a bicontinuous microemulsions, in conjunction with quaternary ammonium salts, as polymerizable surfactant.
In general, it is desirable for the ion conductivities of such materials to be as high as possible. Examples of commercial polymer electrolyte membranes, such as those of Nafion (Dupont) which are perfluorosulfonic polymers, and Dais Corporation, which are non-fluorinated membranes made of triblock copolymers with 50% to 60% sulfonation, show conductivity ranging from 0.01 to 0.1 S/cm.
More recently, Peled et al. (Electrochemical and solid state polymer binder; 1998; 5:210) synthesized ion conducting membranes which incorporated ceramic powder and polymer binder doped with CF3SO3H/H2SO4. Conductivity of the membrane was reported to be about 0.21 S/cm.
Accordingly, it is an object of the present invention to provide copolymerizable surfactant compounds which are stable, economical to produce, and can be reliably used in a bicontinuous microemulsion polymerization process to be polymerized with a suitable copolymer to give rise to ion conducting membranes having high proton conductivities. It is also an object of the invention to develop routes of synthesis for the copolymerizable surfactant compounds and for the membranes that are derivable from the surfactant compounds which are faster and more economical than existing commercial processes.